From now on, a transponder that is contained in a transmission device such as a router is expected to be made smaller in size to approximately 1/10 in terms of an area ratio. In accordance with this, devices (a light source, an optical modulator, a receiver, etc.) that are contained in the transponder are required to be made extremely smaller in size.
Against a background of this, a semiconductor optical device has attracted attention. A semiconductor optical device is a generic name for an optical device that is made of a semiconducting material. Typical examples of the semiconducting material include indium phosphide, silicon, gallium arsenide, and the like.
A semiconductor optical device can be more easily made smaller in size than a conventional optical device. In a case where an optical modulator is taken as an example, a lithium-niobate modulator that is contained in a transponder having a housing of 5 inches×7 inches has a dimension of approximately 5 cm. Meanwhile, a semiconductor optical modulator that is made of indium phosphide or silicon has a dimension of not more than several millimeters. A semiconductor optical modulator thus can have a size that is not more than 1/10 the size of a lithium-niobate modulator.
Meanwhile, a semiconductor optical device has a problem of a mode mismatch between the semiconductor optical device and an optical fiber that is used in a transmission network. Specifically, an optical waveguide (single mode) provided in a semiconductor optical device has a width of approximately several hundred nm to several μm. Meanwhile, an optical fiber has a core diameter of approximately 10 μm. Thus, in a case where optical coupling between the semiconductor optical device and the optical fiber is attempted, a mode mismatch occurs. This causes a great coupling loss.
There is known a technique in which a so-called mode field conversion structure is used and a mode field diameter conversion is carried out between (a) an optical waveguide provided in a semiconductor optical device and (b) an optical fiber so that a coupling loss caused by such a mode mismatch as described earlier is prevented.
Examples of the mode field conversion structure include a lens that is provided on an end surface of an optical fiber. Note, however, that use of the lens increases the number of components and a tolerance in optical coupling is determined in accordance with a mode field diameter of an optical waveguide provided in a semiconductor optical device. In view of this, extremely highly accurate mode field diameter conversion is required. Thus, use of the lens as the mode field conversion structure makes it difficult to achieve mode field diameter conversion that is stable and low in coupling loss.
Under the circumstances, attention is drawn to a technique in which a tapered optical waveguide is provided in a semiconductor optical device and the tapered optical waveguide is used as a mode field converter. Examples of this technique are disclosed in Patent Literatures 1 and 2. According to the techniques disclosed in Patent Literatures 1 and 2, a tapered optical waveguide made of silica (specific refractive index: approximately 1.45) is used as a core and the tapered optical waveguide is surrounded with a clad filled with air (specific refractive index: 1), so that light is allowed to be confined in the tapered optical waveguide.